Electric power distribution systems are commonly exposed to line disturbances, also called transient conditions, current (voltage) transients, and short-circuit conditions. Line disturbances may arise from insulation breakdowns, lightning strikes, or other faults. Traditionally, mechanical circuit breakers have been used to protect electric power distribution systems from the effects of line disturbances. A mechanical circuit breaker uses metal contacts that are mechanically opened in the presence of a transient condition. A mechanical circuit breaker is usually operated from a trip-circuit that measures the magnitude and duration of the system current. When the system current exceeds a predetermined threshold, the mechanical circuit breaker is opened.
The operation of circuit breakers in an electric power distribution system needs to be coordinated. A line disturbance must be isolated or cleared by separating the portion of the power system adjacent to the line disturbance from the remaining portion of the power system. In this way, the line disturbance only effects a portion of the power system.
In such a scheme, down-stream circuit breakers that protect individual equipment and feeder lines are activated first. If the down-stream circuit breakers fail to clear the fault, then up-stream circuit breakers are activated. The up-stream circuit breakers isolate large numbers of feeder lines, power generators, and other important elements of the power system. Thus, activation of up-stream circuit breakers can have a substantial effect on the operation of the entire power system. Consequently, it is highly desirable to rely upon down-stream circuit breakers to clear a line disturbance.
Quality requirements for electric power supply systems have increased dramatically in recent years. Modern automated manufacturing and process controls use complex machinery and data handling equipment that employ sophisticated electronic switching and control circuits. The switching and control circuits are extremely sensitive to supply voltage variations. Small supply voltage variations may cause the switching and control circuits to malfunction or fail. As a result, manufacturing or related operations may be interrupted or suspended. Such interruptions or suspensions result in significant monetary losses in all types of commercial operations.
To solve the problem of supply voltage variations it is necessary to use circuit breakers that can be activated within a fraction of a power signal cycle. Mechanical circuit breakers are not fast enough for this purpose. However, solid-state power semiconductors with sub-cycle response times have recently been developed. Thus, it is possible to rely upon solid-state power semiconductors to isolate line disturbances and thereby prevent supply voltage variation problems.
Solid-state power semiconductors can be used to provide sub-cycle isolation of a down-stream line disturbance. However, if used in this fashion, then the down-stream protective devices are not used. As indicated above, it is desirable to rely upon down-stream protective devices to clear line disturbance. Thus, it would be highly desirable to provide a solid-state circuit breaker that provides sub-cycle isolation of line disturbances, but also allows for the operation of installed down-stream circuit breakers.